Gypsum is the mineralogical name for the chemical compound calcium sulfate dihydrate (CaSO4.2H2O).
By supplying thermal energy, gypsum loses 1 1/2 molecules from its chemically bound water of crystallization, per formula unit, and the calcium sulfate dihydrate is thereby converted into calcium sulfate hemihydrate CaSO4.0.5H2O.
There are two technical forms of calcium sulfate hemihydrate and these are frequently distinguished, in practice, as alpha and beta modification, even though they are identical in terms of chemico-mineralogy. If the thermal energy is supplied at atmospheric pressure, the beta modification of the calcium sulfate hemihydrate is obtained. The grains of such a fired product have a high inherent porosity and consist of innumerable individual crystallites. On account of the high surface area, a binder comprising this hemihydrate has a high water requirement during processing, and this results in a water-gypsum factor of about 0.6-1.0 (determined via gauging quantity; test according to EN 13279-1; September 2005). This means that low strength values are obtained during processing. Beta modification is the main constituent of plaster of Paris which is greatly important as a binder for the production of gypsum mortar and gypsum plasterboard. The present invention concerns the production of the other form of calcium sulfate hemihydrate, alpha modification.
This alpha modification of the calcium sulfate hemihydrate is produced from supersaturated aqueous solutions, specifically in solutions of electrolytes of acids and salts or at elevated temperature and elevated steam pressure in autoclaves. These conversions are generally carried out using additives which have a desirable influence on the morphology of the crystals which are produced.
In contrast to beta modification, alpha modification provides well-formed individual crystals which, after grinding, produce binders having water-gypsum factors of about 0.3 to 0.5. This results in considerably higher strength values during use. Binders such as these are used, for example, in the dental sector.
If alpha-calcium sulfate hemihydrate crystallizes from aqueous solutions or in electrolyte solutions, use is made of the term “wet process”. In wet processes with aqueous solutions, a distinction is made between “neutral” operation and “acidic” operation. Whereas, in neutral operation, roughly pH-neutral gypsums such as natural gypsums or FGD gypsums are converted without setting the pH value (see, e.g., GB 563 019), the pH value is deliberately set to about 2 to 3 using sulfuric acid in acidic operation, where acidic starting gypsums such as phosphogypsum are normally used (DE 11 57 128 A1). If, by contrast, alpha-calcium sulfate hemihydrate is produced in steam-charged autoclaves, use is made of the term “dry process” (see, e.g., U.S. Pat. No. 1,901,051).
The production of alpha-calcium sulfate hemihydrate from electrolyte solutions (U.S. Pat. No. 2,616,128) has not been able to succeed in industry since considerable corrosion occurs on the equipment. In addition, the process-technology steps of dewatering by filtration, filter cake washing and wastewater treatment, which need to be carried out before drying, came across insurmountable difficulties. In addition, instances of material adhesion and encrustation cannot be avoided as a result of set calcium sulfate dihydrate.
In the “dry process”, coarse pieces of natural gypsum having a high degree of purity and a compact crystal microstructure are stacked on rack trolleys or in perforated steel baskets and heated in a steam-charged autoclave. In order to achieve a high degree of conversion even in the core region, it is necessary for the gypsum to reside in the pressurized steam atmosphere for a time period of several hours. The same principle is applied for a “dry process” in which firstly finely-divided gypsum is pressed to form stone blanks and is then subjected to the heat treatment described above in pressurized steam (see, e.g., DE 38 19 652 03).
According to DE 0937276 C, DE 4217978 A1 and EP 0572781 B1, a further approach for a dry or semi-dry process provides a horizontal (or else vertical) agitator autoclave. According to DE 093 7276 C, the first water of crystallization which emerges from the gypsum builds up the pressure in the agitator autoclave, if appropriate assisted by water of imbibition which is present (up to about 3%). Granular material is used, and drying can be carried out in the drum.
According to DE 4217978 A1 and EP 0572 781 B1, finely-divided material is converted in the horizontal agitator autoclave without or with a very small addition of water with and without additives. Drying can be carried out in the autoclave or in a downstream apparatus.
Claim 1 of EP 0572 781 B1 claims a process for the production of alpha-calcium sulfate hemihydrate from calcium sulfate dihydrate using the dry process, in which fine granular raw material is charged and converted under the action of pressure, temperature and steam, this process being characterized in that the material is mixed in the interior of a stationary reactor using externally driven mixing devices or in the interior of a rotating reactor using stationary or movable mixing devices, and in that the temperature of the material in the reactor is measured continuously and controlled in accordance with a preselected time curve.
Furthermore, the dependent claims substantially claim that the raw material is charged having a specific proportion of surface moisture and/or having a reduced proportion of water of crystallization, that additives are added, that the pressure is controlled in accordance with a preselected time curve, that steam is withdrawn and air is admixed, and that a separate downstream drying device is provided.
The description also refers to appropriately designed mixing tools which are intended to prevent agglomeration (column 4, lines 15-19). However, a corresponding, detailed disclosure in this respect cannot be found in EP 0572 781 B1.
To date, it has not been possible to successfully operate installations according to this disclosure. A model installation was taken down after two years of test operation due to insurmountable technical difficulties.
The revocation of the corresponding patent came into legal force on Jul. 23, 1998.